Teaching English (San José)

Hi, I'm hoping to move to Costa Rica to teach English medium-term once I finish my degree (Politics) in London. Does anyone have advice on where best to look for a placement? I'm hoping to live in a city, but would love to be able to go to the beach or do some exploring in my spare time. Also, I would preferably be looking for somewhere with quite a large English speaking ex-pat population to help ease my transition.